In subambient temperature separations, refrigeration is provided to a gas mixture to maintain the low temperature conditions and thus facilitate the separation of the mixture into its components for recovery. Examples of such subambient temperature separations include cryogenic air separation, natural gas upgrading, hydrogen recovery from raw syngas, and carbon dioxide production. One way for providing the requisite refrigeration to carry out the separation is by turboexpanding a fluid stream and using the refrigeration generated by the turboexpansion, either directly or by indirect heat exchange, to facilitate the separation. Such a system, while effective, uses significant amounts of energy and can reduce product recovery and is thus costly to operate.
Refrigeration can also be generated using a refrigeration circuit wherein a refrigerant fluid is compressed and liquefied and then undergoes a phase change at a given temperature from a liquid to a gas thus making its latent heat of vaporization available for cooling purposes. Such refrigeration circuits are commonly used in home refrigerators and air conditioners. While such a refrigeration circuit is effective for providing refrigeration at a given temperature and at relatively high subambient temperatures, it is not very efficient when refrigeration at low temperatures and over a relatively wide temperature range is desired.
Accordingly it is an object of this invention to provide a method for carrying out a subambient temperature separation of a fluid mixture, especially one carried out at cryogenic temperatures, more efficiently than with conventional separation systems and without the need for using turboexpansion to generate any of the requisite refrigeration for the separation.